Within larger geographical areas, that are covered by regional mobile radio communication systems, there is a need for local mobile radio communication systems that cover areas that comprise only a smaller part of the area covered by the regional mobile radio communication system. The local system can for instance be a system that covers essentially only an office building, where a company that uses the office building has a private branch exchange communicating with pocket phones over local base stations instead of a conventional exchange with stationary telephone sets connected to the exchange with cables. The base stations of the local system are in this case arranged at a much higher density and have a shorter range than the regional system. The pocket phones also have a much shorter range. Furthermore, the pocket phones are not moved as rapidly as mobile stations in the regional system. The pocket phones can therefore be designed simpler, more inexpensive and smaller than mobile stations in the regional system.
However, when the regional and local mobile radio communication systems are present in the same area, a frequency shortage arises if they are supposed to have separate frequency bands. On the other hand, if they are allowed to have the same frequency band, there is a risk that the local system will disturb the regional system.
From U.S. Pat. No. 4,790,000 a combined mobile radio communication system is previously known, in which a regional cellular mobile telephony system coextists with a local private mobile telephone system located within the regional mobile telephony system. In the previously known combined mobile radio communication system, the regional mobile telephony system is divided into several large areas, each such large area being further subdivided into several cells. Within each cell a certain number of radio channels with given frequencies are used. The radio channels in the different cells in a large area have different frequencies. However, these frequencies are repeated between the large areas in accordance with a certain pattern. This arrangement prevents the different cells from disturbing each other since the distance between the cells in question is sufficiently large. The local mobile telephony system is located in a cell within a large area and also uses the same radio channels as the cells in the large area. However, the local mobile telephony system is prevented from using the same radio channels as that cell in which the system is located. Due to the local character of the local mobile telephony system and the implied low signal strengths, the system will not disturb neighbouring cells in spite of the fact that the same radio channels are used. A drawback of this previously known system, however, is that it is not possible to let both the local and regional systems use the same radio channels in that cell in which the local system is located. A further drawback is that the frequencies that have been allocated to the local system will have to be changed if the frequency plan of the regional system is changed or if a cell splitting is performed in that system.